If you have constructed a digital wattmeter based on one of the Analog
Devices AD8307 logarithmic amplifiers you can probably easily interface it
to a PC. The design by OZ2CPU on http://www.webx.dk is capable of this and
the software in the PIC sends a continuous stream of power readings in
tenths of a dB to the serial port.

PolarPlot V3.1 (http://www.g4hfq.co.uk/plphelp/plphelp.htm) can now use
these power readings to plot the radiation pattern of beam antennas.

The advantage over the previous method using a sound card is that much
better resolution can be achieved at levels lower than -35dB. In addition it
is easier to set up levels and there is no more battling with the Windows
mixer application or the RF gain of your receiver to get the audio level
correct.

V3.1 of PolarPlot still supports the sound card as well as the OZ2CPU
wattmeter. A purpose-built wattmeter is being designed and programmed by
M0DFT.

73 de Bob, G4HFQ